All Glossary

Reactive Glazes

In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.

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Details

Reactive glazes develop variations in color, opacity or texture rather than the uniform surface typical of a porcelain sink or toilet bowl. Their mottling, streaking, speckling and color changes can result from several mechanisms, including crystal growth, phase separation, speckling agents and interactions between layered glazes. Differences in glaze thickness and the underlying clay body can further enhance these effects. Many reactive glazes have fluid melts that encourage movement and interaction during firing, but a glaze does not have to run to produce variegation. The recipe, application and firing schedule—especially cooling—work together to determine the result.

You can develop your own reactive glazes by adding variegators, opacifiers and colorants to a base glaze you already understand. Rutile and titanium dioxide are common variegators; zircon and tin oxide provide opacity; metal oxides and ceramic stains provide color. These materials interact with the base chemistry, so the same addition can produce very different results in different glazes. Start with small test batches, vary additions systematically and record application thickness and firing conditions. To explore an effect you admire in a commercial glaze, search for its general type and firing temperature—for example, “rutile blue glaze recipe cone 6.” Published recipes can provide starting points; use a recipe logic approach to screen candidates before testing them.

When a reactive glaze has a fluid melt, controlling its movement is part of making it usable. Application thickness, layering, firing conditions and placement on the piece all affect how far it runs. A catch glaze, a less fluid glaze positioned below the running glaze, can help arrest the flow. However, the combination needs testing on vertical surfaces before committing the glaze to finished ware.

Related Information

Toilet bowl glaze vs. variegated glazes

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Most artists and potters want some sort of visual variegation in their glazes. The cone 6 oxidation mug on the right demonstrates several types. Opacity variation with thickness: The outer blue varies (breaks) to brown on the edges of contours where the glaze layer is thinner. Phase changes: The rutile blue color swirls within because of phase changes within the glass (zones of differing chemistry). Crystallization: The inside glaze is normally a clear amber transparent, but because these were slow-cooled in the firing, iron in the glass has crystallized on the surface. Clay color: The mugs are made from a brown clay, the iron within it is bleeding into the blue and amplifying color change on thin sections.

High melt fluidity is required for the visual effect of this glaze

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This is G3948A, an iron red cone 6 reactive glaze (similar to several commercially available products). The reason for the variegated surface is the high fluidity of the melt. Adequate thickness is also important, enabling it to run downward to some extent. That means this is not actually over-fired. Using it thus requires consideration of the running behavior, accommodating it in the shape of the ware on which it is used. Obviously, using this on the insides of pieces would result in pooling at the base, which would likely produce glaze compression, cracking the piece during cooling. Use on the outsides may require a catch glaze.

Fluid melt glazes need an uneven surface...

To develop visual interest

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This is the same glaze on the outside of these two pieces. It develops the variegated deep blue character only when thick. But if it were applied thick enough on the left piece, it would run off onto the kiln shelf. However, the recesses in the texture-rolled surface of the one on the right have caught the flow, creating the thicknesses needed to get the color. Another factor is that the piece on the right is buff stoneware. Thus, the clay contains some iron, and it is bleeding into the glaze to help develop the color.

Three visual glaze mechanisms make this piece unique

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"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or the firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954N black engobe on the inside and partway down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through, but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).

Titanium Dioxide in a cone 6 calcia matte glaze

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The glaze is G1214Z1 cone 6 base calcia matte on Plainsman M390 fired at cone 6 using the PLC6DS schedule. 5% titanium dioxide has been added (a high percentage). Titanium can create reactive glazes, like rutile, with no other colorants added. This effect also works well on matte surfaces, but the glaze needs good melt fluidity (that is good because functional mattes melt well). Calcia mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide, lighter colors and better blues are possible compared to rutile (iron is still needed, but it is coming from the body here). Like rutile, the effects are dependent on the cooling rate of the firing, slower cools produce more reactivity. Even application without drips is important (mixing as a thixotropic dipping glaze is best). This appearance also depends on using a dark-burning body or engobe.

Layering glazes to get variegation

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Example of the variegation produced by layering a matte white glaze of stiffer melt (having lower melt fluidity) over a darker glossy glaze of more fluid melt. This was fired at cone 6. The body is a slip-cast stoneware and the glazes employ calcium carbonate to encourage LOI bubbling during melting (each bubble reveals the color and texture of the underlying glaze layer). It is also possible to get this effect using the same base glaze (but stained with different colors).

The rutile mechanism in glazes

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2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If too much rutile is added (as low as 6%) the surface can turn an ugly yellow in a mass of titanium crystals.

Tipping-Point Glazes are Good. And Bad.

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Reactive glazes don't melt into a homogeneous melt, and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) creates a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable (or at least less controllable) effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the oxides, kiln atmosphere, cooling or heating cycle (firing schedule), etc. These glazes are at their best when each piece has a unique, artistic character. But this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and less understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

Making Ravenscrag Floating Blue dance more at cone 8

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Here GR6-M it is fired to cone 8 where the melt obviously has much more melt fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course, it is running a lot more, so caution will be needed.

Variegation and phase separation with about 5% rutile

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The glaze is a dolomite matte. G2571A, fired to cone 10R. High-fire reduction is among the best processes to exploit the variegating magic of rutile.

Boron blue in low-fire transparent glazes

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This high-boron high-CaOcone 04 glaze is generating calcium-borate crystals during cool-down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO in favor of a lower-melting oxide, that will certainly help. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier (among the most expensive of all materials).

An iron red cone 6 reactive glaze up close

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G3948A is a cone 6 iron red. This sample is firing using the C6DHSC schedule. It is a reactive glaze in more ways than one. This closeup reveals just how much is happening on that fired surface. The recipe contains spodumene, an expensive material, but clearly it is worth it.

A pottery glaze that can eat through a firebrick!

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The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, the 50:30:20 historically popular Gerstley Borate base recipe used for reactive and transparent glazes. The B₂O₃ in that glaze was 3x the normal amount in cone 6 glazes, making it so melt fluid it can eat through a firebrick!

The glaze on the right is G2926A3, an adjusted version that cuts the B₂O₃ level and adds SiO₂ (from silica and nepheline). The result is more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue: High B₂O₃ is not the key; my adjustment lowers it significantly. CaO/MgO is also lower, so that is not the key. The SiO₂ appears to be the enabler here; it is much higher (from 2.6 to 3.48, a hugh increase). And, I am using 325 mesh silica, so it dissolves in the melt better, delivering even more SiO₂. Boron blue thus seems to thrive on enough SiO₂ coupled with high B₂O₃ and low Al₂O₃ with some MgO/CaO.

In pursuit of a reactive cone 6 base that I can live with

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These GLFL tests and GBMF tests for melt-flow compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are:
G3814 - Moderate zinc, no boron
G2938 - High-soda+lithia+strontium
G3808 - High boron+soda (Gerstley Borate based)
G3808A - 3808 chemistry sourced from frits
G3813 - Boron+zinc+lithia
G3806B - Soda+zinc+strontium+boron (mixed oxide effect)
This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.

Cone 6 oil-spot glaze effect, what works and does not work?

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Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at insight-live.com, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.

Cone 10R variegation and crystal magic

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This is an example of crystallization in a high MgO matte. MgO normally stiffens the glaze melt, forming non-crystalline mattes, but at cone 10R many cool things happen with metal oxides, even at low percentages. Dolomite and talc are the key MgO sources.

Lithium and the dramatic surface of AMACO PC-32 Albany Slip Brown

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Left is G3933A; right is AMACO PC-32 Albany Slip Brown. PC-32 shows much greater movement and a stronger contrast between creamy areas and rich brown. AMACO identifies lithium carbonate in this product, and our own tests show that adding lithium carbonate to a related glaze can greatly increase melt fluidity and transform its appearance. That supports lithium’s importance here, although it does not reveal AMACO’s base recipe or lithium percentage.

Where PC-32 collects more thickly, it develops a lighter, more opaque appearance; over raised contours, it thins and breaks brown. Crystallization may contribute to the cream-colored areas, but opacity as a product of phase separation is another possible mechanism. This raises an interesting development challenge: Can a lithium-free formulation reproduce both the flowing melt and the distinctive cream–brown separation? With the cost of lithium materials it is easy to imagine development work is being done.

Inbound Photo Links

20x5 Cone 6 glossy black with GR6-H, GA6-F glazes over: Crawling will happen unless mitigations are taken

An example of variegation on a tile surface that occurred when using raw manganese dioxide

Cone 10 Reduction, the home of an amazing oxide: Iron

Variegating effect of sprayed-on layer of titanium dioxide

Variegation gone too far!

Same iron red glaze on black stoneware and on porcelain

Glazing for the photo instead of the piece. Is this a “science project” or a functional glaze?

Does ChatGPT know the recipe of this commecial glaze? Yes, it thinks it does. And its reasoning is sound.

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